1. Field of the Invention
The present invention relates to a method for the purification of waste gas of an apparatus for drying biological matter. It furthermore relates to an apparatus for the purification of waste gas of an apparatus for drying biological masses.
2. Description of the Prior Art
In industrial plants, in which for instance wood or feed stock is dried, three groups of air contaminating matter is released, which groups depend from the kind of the biological mass being processed, from the design of the drying apparatuses present, and from the temperature level of the drying process. These three groups of air contaminants are a) dust, b) resin aerosols which the art also defines as blue smoke, and c) organic gases such as formaldehyde, pinenes, terpenes and organic acids.
These contaminants must be removed from the waste gas of drying apparatuses before the waste gas is released into the environment.
The purification treatment necessary for this task substantially depends from the differing portions of the above named groups of contaminants and, therefore, is to be made to suit the respective conditions encountered.
For the purification of the waste gases of drying apparatuses of wood drying plants, specifically three kinds of purifying methods and purifying apparatuses, respectively, have become known such as follows.
a) Wet-washing apparatuses with or without an addition of chemicals and with a discharge of waste water. These known apparatuses are designed as a Venturi-apparatus, a counter-current jet apparatus or also as a wet electro-filter apparatus. These apparatuses are in a position to remove dust at degrees of efficiency of 80-90%. However, regarding organic gases (e.g. formaldehyde) they are restricted to a range of 10-20% regarding the removal efficiency.
b) Regenerative thermal after-burning apparatuses including pre-washing. By means of such apparatuses it is indeed possible to remove dust and also organic gases with an efficiency of more than 90%; such after-burning apparatuses are, however, extremely expensive regarding investment as well as operation.
c) Apparatuses having a combined purification process, including a pre-washing apparatus and a bio-washing apparatus. Such apparatuses, however, specifically due to the problems regarding the removal of resin aerosols and the high points of condensation of the waste gas, have been installed rather seldom. Generally, it is indeed possible to achieve removal efficiency of dust of 80-90%, however only in case that no resin aerosols are present, and regarding organic gases a removing efficiency of not more than 50-60% can be reached.